Wednesday, 21 September 2011

The Cost of Destroying Bitcoin

As is fairly well accepted, if you own half of the global bitcoin mining power, you are in a position to do all sorts of terrible things such as performing double spends with arbitrarily many confirmations. Fortunately, the existing computation power is truly vast, so this attack would be completely infeasible, it is said. But exactly how much does it actually cost?

The Numbers

Currently the compute power of the network is 13.5 THash/s, according to bitcoincharts. Looking at the hardware comparison page on, a good all-round graphics card with "easy" availability is the 6950, which will give you 1.4 MHash/s per dollar spent on the hardware. So assuming there's a really big supply of them, it will cost you around 10M USD to buy the 40k graphics cards to equal the global hashrate. Maybe you'll get some kind of discount on a bulk purchase. On the other hand, you've got to plug them all together and find a big shed in Siberia to house them, so lets for arguments sake just call it 10M USD total.

Some Silliness

How many people have that much money? According to wikipedia there are around 40M millionaires and 1,000 billionaires. Doing a log-log interpolation, I reckon there are around 30,000 people in the world with access to enough capital to bring down bitcoin. But no one except the completely insane would spend their entire fortune on killing some unknown geek project, so perhaps this doesn't matter. However, for the 1,000 billionaires in the world it would only cost 1% of their net wealth.

For context, here are a few things bought over the years by billionaires:

  • David Brooks spent 10M USD on his daughter's 13th birthday party
  • Paul Allen's yacht: 100M USD
  • Roman Abramovic bought Ukrainian footballer Andriv Shevhenko for 30M EUR a few years back despite the Chelsea manager not even wanting him.
  • The domain name was bought for 10M USD
  • 10M USD will buy a middle-of-the-road private jet.
If you're the kind of person who suspects that certain governments would dearly love to put a stop to bitcoin, how about these numbers:
  • F-22 Raptor apparently costs 150M USD. Even a bargain-basement Saab Gripen will set you back 35M USD.
  • 10M USD will buy about 15 cruise missiles.
  • 20M EUR is the amount Robert Bourgi claims to have delivered to Jacques Chirac in duffle bags over the years from african dictatorships.
  • NSA plans to spend 1B USD on a new data centre.
  • etc etc
More Numbers

I believe you can do better than this. At the current difficulty, a 6950 will mine a new bitcoin every 4 days, or 90 in a year. Subtract electricity costs and assume the current bitcoin price continues, you'll make $2 / btc after electricity, meaning that each 6950 will have mined back all of its purchase cost in just over a year. The big question here is how profitable will mining be as you add more and more nodes. Will less efficient miners drop out as you raise the difficulty bar, or will they cling-on regardless? Perhaps by the combination of a slight price rise, say to $8 / bitcoin combined with some thinning-out of creaky old mining rigs, your investment in graphics cards would have actually paid for itself in a year and a half, leaving you free to trash the blockchain at zero cost.

I, for one, am quite concerned about all this.

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